问题 选择题

如图是一种自动测定油箱内油量多少的装置,R1是滑动变阻器,它的金属滑片是杠杆的一端,从油量表指针所指的刻度,就能知道油箱内油量的多少。则下列说法不正确的是(   )

A.油量表是由电流表改装而成

B.油量增加,R1的电阻值减小,油量表指针偏转变大

C.油量减少,R1的电阻值增大,油量表指针偏转变大

D.定值电阻R2可以起到保护电路的作用

答案

答案:C

题目分析:如图,该装置涉及到浮力、杠杆、滑动变阻器、电流表等相关知识,油面上升通过浮子和杠杆使滑动变阻器连入的电阻变小,电路中的电流变大;油面下降通过浮子和杠杆使滑动变阻器连入的电阻变大,电路中的电流变小;逐一进行分析判断。油面降低,浮标下降,浮标推动指针上移,滑动变阻器R连入电路的电阻变大,由欧姆定律可知电路中的电流变小,则电流表指针向左偏转;当油箱内一点油也没有时,油量表的示数为0;但是整个电路中还有电流,故电流的示数不为0;因为电流增大时电流表指针向右偏转,所以,油面越高时,示数越大;即越往右示数越大;油箱内的油越少时,浮标推动指针上移,滑动变阻器R连入电路的电阻变大,由欧姆定律可知电路中的电流变小。

故选C。

点评:当油面升降时,看准滑动变阻器的阻值如何变化是本题的关键所在.灵活运用滑动变阻器的变阻原理:通过改变连入电阻丝的长度,改变阻值的大小,从而改变电路中的电流。

阅读理解

阅读理解。

     Wearing the wrong type of glasses will do harm to your eyes. This is not true for adults, although

incorrect glasses may not be good for children under 10. Ready-made glasses, and not wearing when

you should, won't do harm to your eyes, but you may see better with glasses that are specially made for

you.

     Watching too much television is bad for your eyes. This is not true. People with easily affected eyes

may find they get red and aching from staring at a fixed distance for long periods, but there won't be

any lasting effects.

     Carrots will help you see in the dark. Carrots are rich in beta-carotene, which the body can change

into vitamin A, too little of which can cause night blindness. However, people in most countries don't

need to worry about vitamin A deficiency (缺乏), night blindness is more likely to be connected with

another vision problem.

1. According to the passage, which of the following is TRUE?

A. Ready-made glasses will do harm to one's eyes.

B. Wearing the wrong glasses may do harm to children's eyes.

C. The human body is unable to change beta-carotene into vitamin A.

D. Watching less television may do harm to children's eyes.

2. Night blindness is most likely caused by ______. 

A. staring for long periods 

B. too much beta-carotene 

C. not enough vitamin A 

D. vision problems 

3. According to the article, eye problems can be corrected by ______. 

A. wearing specially-made glasses 

B. performing eye exercises 

C. cutting back on television 

D. eating more carrots 

4. Watching too much television can cause ______. 

A. night blindness 

B. lasting eye problems 

C. a number of vision problems 

D. eye trouble for a limited time 

5. The main purpose of this article is to ______. 

A. tell people of the importance of their eyes 

B. tell people how to protect their eyes 

C. tell people to have more carrots 

D. tell people something more about their eyes 

多项选择题